Alpha2-adrenergic receptors (alpha2-AR) are members of a large family of receptors which transduce their cell signals through heterotrimeric guanine nucleotide binding proteins. Utilizing the alpha2-adrenergic receptor system as a representative subfamily of G-protein coupled receptors, our goal is to define factors that determine signal specificity in the intact cell. The objective of the proposed studies is to define the role of R-G affinity and/or stoichiometry in determining 1) the transduction pathway selected by each receptor subtype and 2) the efficiency of receptor coupling. SPECIFIC AIMS 1) Determine the effects of altered R-G stoichiometry on receptor-effector coupling. 2) Determine if competition between G-proteins for receptor recognition influences signal propagation. 3) Determine if the existence of receptor reserve and partial agonism depends on the receptor subtype. 4) Determine the role of the type and amount of G-protein recognized by the receptor in the existence of receptor reserve and partial agonism. To achieve these aims, receptor-effector coupling is analyzed following alteration of R-G stoichiometry by a) cotransfection of NIH-3T3 cells with the receptor gene and C-protein cDNA b) by transcriptional activation of the receptor gene in cells expressing endogenous receptor gene c) progressive reduction in receptor density by receptor alkylation d) progressive reduction of G-alpha available for receptor coupling. Utilizing both full and partial agonists, analysis of receptor-effector coupling involves three standard evaluations: a) high affinity agonist binding b) GTP-gamma-35S binding c) adenylylcyclase activity d) identification of G-proteins coupled to the receptor subtypes. The results of these studies will provide important information as to factors that determine signal transduction specificity for this class of membrane receptors. Data generated from these experiments should facilitate current efforts to define and understand partial agonism and efficacy. The results generated have broad applicability and should contribute to the development of therapeutic approaches that target the R- G or G-E interface as opposed to the receptor's binding site.